When we first see Harry Goldfarb (Jared Leto), he's persuading his frightened, widowed mother (Ellen Burstyn, pictured) to let him pawn her TV set, in order to pay for his drug habit.

He and his best friend Tyrone C. Love (Marlon Wayans) see drugs as an easy way to feel good and make money. And Harry's not such a bad boy. As soon as he has made a few dollars out of drug-dealing, he wants to buy his mother a new TV set.

Harry has an attractive new girl-friend (Jennifer Connelly), whom he genuinely loves and whose dreams to become a dress designer he wants to encourage. They use drugs to iron out any arguments that may arise in their relationship, and cushion themselves from painful memories.

Harry may not look after himself too well - there's a nasty sore on his arm where he keeps injecting himself - but he cares enough about others to try to persuade his mother to stop using diet pills.

Gradually, however, all four of the leading characters plunge into nightmare. The mother starts hallucinating that she is being attacked by her refrigerator, wanders the subway and ends up in a mental hospital. Her son and his friend end up disfigured and imprisoned respectively, while the girl-friend loses her self-respect and ends up selling herself to pornographers.

Not every great film is easy to watch. Requiem for a Dream contains scenes that are as harrowing as any I've seen, but they're there for a purpose. You could describe it as a cautionary tale about drugs, but that's a bit like saying that My Fair Lady is tuneful, or Albert Einstein knew a thing or two.

The film may sound very depressing, and it certainly doesn't pull any punches. But the film gets inside each of the character's mind so skilfully that you really care for these people.

Early on, Aronofsky makes drug-taking look quick, cool and even funny, through montages of advertising-style close-ups. But as the film goes on, we see more of the gruesome reality behind the ritual, in scenes that make one feel almost physically sick.

This is a film with wonderful visual creativity, a rare ear for sound and music (Clint Mansell's score, a blend of string quartet and modern electronics, is itself a masterpiece) and an emotional power that mark out Darren Aranofsky as one of the most exciting writer-directors working today.

He has the expressionist ability of the great German silent directors, coupled with the flair behind the camera of Martin Scorsese and David Fincher. And although his directorial style makes much use of distorting lenses and variations in camera speed, he can also draw the best out of his actors.

When much younger, Ellen Burstyn won an Oscar for Alice Doesn't Live Here Any More; and she was a major reason why The Exorcist was so successful - her performance as Linda Blair's anguished mother grounded the horror in believable behaviour.

Her performance here is only one of four central roles, but it makes every other performance by an actress in the last few years look superficial. This is the most affectionate display of motherhood since Brenda Blethyn's in Mike Leigh's Secrets and Lies; but she also presents the cinema's most memorable depiction in years of a soul spiralling downwards into madness. And it is because we care about her that the mental hospital scenes at the end make the ordeal that Jack Nicholson went through in One Flew Over The Cuckoo's Nest look tame.

The script, by Aronofsky and Last Exit To Brooklyn author Hubert Selby is so cleverly crafted that each step towards Hell appears grounded not in the moralising of the authors but in the weaknesses of each character. All the actors descend into their own particular form of degradation, without having to resort to overwrought posturing.

Burstyn will deservedly win the lion's share of the rave notices; but Jennifer Connelly deserves special praise as well.

I remember spotting her as a child actress in Once Upon a Time In America. 17 years on, she emerges here as a brave and considerable talent. Her sensitive study of self-hatred is never showy, but it's finely observed and deeply felt; and she courageously submits herself to scenes at the end that give the lie to the comfortable notion that one man's pornography is another person's harmless fun.

A lot of credit should go to the director. There's none of the cruelty and voyeurism that made me so uneasy about Lars von Trier's studies of female breakdown, in Breaking the Waves and Dancer in the Dark.

This film is not for the faint-hearted. Like Todd Solondz's Happiness and David Fincher's Fight Club, it dares to examine areas that are undoubtedly shocking. Yet non-exploitative films should be able to tackle such topics in a responsible manner.

Aronofsky's film is not only a remarkable advance on his first, Pi ; it's the best movie I have seen about addiction, capturing both the allure of drugs and their direst consequences. It is the definitive film about drug abuse that Trainspotting tried and failed to be.

The horrific sights and the occasional use of foul language made an 18 certificate inevitable; but there's a side of me that believes this exceptional film should be shown to every 13 year-old, so that they can see what drug addiction really involves.

And anyone who cares about serious film-making should support this movie. Okay, this is only January, but I'm confident that there won't be a better, or more truthful, picture this year.